Add Use-MdbcTransaction.

.build.ps1

@@ -220,12 +220,22 @@ task PushNuGet NuGet, {
 Clean
 
 # Synopsis: Make and push the PSGallery package.
-task PushPSGallery Package, Version, {
-	assert ($TargetFramework -eq 'netstandard2.0')
-	$NuGetApiKey = Read-Host NuGetApiKey
-	Publish-Module -Path z/tools/$ModuleName -NuGetApiKey $NuGetApiKey
-},
-Clean
+task PushPSGallery @(
+	if ($TargetFramework -eq 'netstandard2.0') {
+		'Package'
+		'Version'
+		{
+			$NuGetApiKey = Read-Host NuGetApiKey
+			Publish-Module -Path z/tools/$ModuleName -NuGetApiKey $NuGetApiKey
+		}
+		'Clean'
+	}
+	else {
+		{
+			Invoke-Build PushPSGallery -TargetFramework netstandard2.0
+		}
+	}
+)
 
 # Synopsis: Test synopsis of each cmdlet and warn about unexpected.
 task TestHelpSynopsis {

Module/en-US/Mdbc.dll-Help.ps1

@@ -312,7 +312,12 @@ $AClient = @{
 
 $ASession = @{
 	parameters = @{
-		Session = 'Specifies the client session which executes the command.'
+		Session = @'
+Specifies the client session which invokes the command.
+
+If it is omitted then the cmdlet is invoked in the current default session,
+either its own or the transaction session created by Use-MdbcTransaction.
+'@
 	}
 }
 
@@ -1156,3 +1161,56 @@ Tells to ignore document elements that do not match the properties.
 '@
 	}
 }
+
+### Use-MdbcTransaction
+Merge-Helps $AClient @{
+	command = 'Use-MdbcTransaction'
+	synopsis = 'Invokes the script with a transaction.'
+	description = @'
+The cmdlet starts a transaction session and invokes the specified script. The
+script calls data cmdlets and either succeeds or fails. The cmdlet commits or
+aborts the transaction accordingly.
+
+The transaction session is default for cmdlets with the parameter Session.
+For the script the session is exposed as the automatic variable $Session.
+
+Nested calls are allowed but transactions and sessions are independent.
+In particular, they may not see changes made in parent or nested calls.
+'@
+	parameters = @{
+		Script = @'
+Specifies the script to be invoked with a transaction session.
+'@
+	}
+
+	outputs = @(
+		@{
+			type = '[object]'
+			description = 'Output of the specified script.'
+		}
+	)
+
+	examples = @(
+		@{
+			code = {
+				# add several documents using a transaction
+				$documents = ...
+				Use-MdbcTransaction {
+					$documents | Add-MdbcData
+				}
+			}
+		}
+		@{
+			code = {
+				# move a document using a transaction
+				$c1 = Get-MdbcCollection MyData1
+				$c2 = Get-MdbcCollection MyData2
+				Use-MdbcTransaction {
+					# get and remove from MyData1 | add to MyData2
+					Get-MdbcData @{_id = 1} -Remove -Collection $c1 |
+					Add-MdbcData -Collection $c2
+				}
+			}
+		}
+	)
+}

Src/Commands/AbstractSessionCommand.cs

@@ -2,9 +2,9 @@
 // Copyright (c) Roman Kuzmin
 // http://www.apache.org/licenses/LICENSE-2.0
 
-using MongoDB.Bson;
 using MongoDB.Driver;
 using System;
+using System.Collections.Generic;
 using System.Management.Automation;
 
 namespace Mdbc.Commands
@@ -15,15 +15,31 @@ public abstract class AbstractSessionCommand : Abstract, IDisposable
 		bool _disposed;
 		IClientSessionHandle _Session;
 
+		//! ThreadStatic and `= new Stack()` fails in Split-Pipeline
+		[ThreadStatic]
+		static Stack<IClientSessionHandle> _DefaultSessions_;
+		static Stack<IClientSessionHandle> DefaultSessions { get { return _DefaultSessions_ ?? (_DefaultSessions_ = new Stack<IClientSessionHandle>()); } }
+		internal static void PushDefaultSession(IClientSessionHandle session) { DefaultSessions.Push(session); }
+		internal static IClientSessionHandle PopDefaultSession() { return DefaultSessions.Pop(); }
+
 		[Parameter]
 		public IClientSessionHandle Session
 		{
 			get
 			{
 				if (_Session == null)
 				{
-					_Session = MyClient.StartSession();
-					_dispose = true;
+					if (DefaultSessions.Count == 0)
+					{
+						// temporary session
+						_Session = MyClient.StartSession();
+						_dispose = true;
+					}
+					else
+					{
+						// the current default session
+						_Session = DefaultSessions.Peek();
+					}
 				}
 				return _Session;
 			}

Src/CollectionExt.cs → Src/Commands/CollectionExt.cs

@@ -6,14 +6,14 @@
 using MongoDB.Driver;
 using System.Collections.Generic;
 
-namespace Mdbc
+namespace Mdbc.Commands
 {
 	static class CollectionExt
 	{
 		public static long MyCount(this IMongoCollection<BsonDocument> collection, IClientSessionHandle session, FilterDefinition<BsonDocument> filter, long skip, long first)
 		{
 			if (skip <= 0 && first <= 0)
-				return collection.CountDocuments(filter);
+				return collection.CountDocuments(session, filter);
 
 			var options = new CountOptions();
 			if (skip > 0)

Src/Commands/UseTransactionCommand.cs

@@ -0,0 +1,48 @@
+
+// Copyright (c) Roman Kuzmin
+// http://www.apache.org/licenses/LICENSE-2.0
+
+using MongoDB.Driver;
+using System;
+using System.Collections.Generic;
+using System.Management.Automation;
+
+namespace Mdbc.Commands
+{
+	[Cmdlet(VerbsOther.Use, "MdbcTransaction"), OutputType(typeof(IMongoDatabase))]
+	public sealed class UseTransactionCommand : AbstractClientCommand
+	{
+		[Parameter(Position = 0, Mandatory = true)]
+		public ScriptBlock Script { get; set; }
+
+		protected override void BeginProcessing()
+		{
+			var session = Client.StartSession();
+			AbstractSessionCommand.PushDefaultSession(session);
+			try
+			{
+				session.StartTransaction();
+				try
+				{
+					var vars = new List<PSVariable>() { new PSVariable("Session", session) };
+					var result = Script.InvokeWithContext(null, vars);
+					foreach (var item in result)
+						WriteObject(item);
+
+					session.CommitTransaction();
+				}
+				catch (RuntimeException exn)
+				{
+					var text = $"{exn.Message}{Environment.NewLine}{exn.ErrorRecord.InvocationInfo.PositionMessage}";
+					var exn2 = new RuntimeException(text, exn);
+					WriteException(exn2, null);
+				}
+			}
+			finally
+			{
+				AbstractSessionCommand.PopDefaultSession();
+				session.Dispose();
+			}
+		}
+	}
+}

Src/GlobalSuppressions.cs

@@ -3,4 +3,4 @@
 [assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1819:Properties should not return arrays", Justification = "rk", Scope = "namespaceanddescendants", Target = "Mdbc.Commands")]
 [assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1825:Avoid zero-length array allocations.", Justification = "<Pending>", Scope = "member", Target = "~F:Mdbc.MyDocument.EmptyArray")]
 [assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1010:Collections should implement generic interface", Justification = "<Pending>", Scope = "type", Target = "~T:Mdbc.Collection")]
-[assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1010:Collections should implement generic interface", Justification = "<Pending>", Scope = "type", Target = "~T:Mdbc.Dictionary")]
+[assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Design", "CA1010:Collections should implement generic interface", Justification = "<Pending>", Scope = "type", Target = "~T:Mdbc.Dictionary")]

Src/Mdbc.csproj

@@ -16,7 +16,7 @@
     <PackageReference Include="PowerShellStandard.Library" Version="5.1.0" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="2.9.7">
+    <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="2.9.8">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>

Tests/About.test.ps1

@@ -127,5 +127,6 @@ task PublicTypes {
 		'RenameCollectionCommand'
 		'SetDataCommand'
 		'UpdateDataCommand'
+		'UseTransactionCommand'
 	)
 }

Tests/MongoFiles.test.ps1

@@ -141,11 +141,11 @@ task Get-MongoFile Update-MongoFiles, {
 	$r = @(Get-MongoFile -CollectionName test 'collectionext|documentinput' | Sort-Object)
 	$r
 	equals 3 $r.Count
-	assert ($r[0] -clike '*\Mdbc\Src\CollectionExt.cs')
+	assert ($r[0] -clike '*\Mdbc\Src\Commands\CollectionExt.cs')
 	assert ($r[1] -clike '*\Mdbc\Src\DocumentInput.cs')
 	assert ($r[2] -clike '*\Mdbc\Tests\DocumentInput.test.ps1')
 
 	$r = @(Get-MongoFile -CollectionName test 'CollectionExt.cs' -Name)
 	equals 1 $r.Count
-	assert ($r[0] -clike '*\Mdbc\Src\CollectionExt.cs')
+	assert ($r[0] -clike '*\Mdbc\Src\Commands\CollectionExt.cs')
 }